FIG. 1 shows a known circuit arrangement having a resonant output characteristic. The inductor L regulates the current from a pulse source Ue. The AC current limited by the inductor is rectified by a bridge rectifier and filtered by a capacitor.
In practice, however, with such a circuit arrangement there are problems regarding electromagnetic compatibility. The problem is manifested in customary arrangements of semiconductor light sources such as LEDs as load. The latter are usually arranged on thin printed circuit boards that are in turn implemented on a metallic heat sink for cooling the LEDs. The heat sinks are normally grounded for safety reasons. In this case, parasitic capacitances CModule occur between the LEDs and the grounded heat sink. Said parasitic capacitances may be very high with a value of up to 2 nF. On account of the high parasitic capacitances, the interference potential is correspondingly high as well.
FIG. 2 shows a real test set-up similar to the circuit described in FIG. 1 on the basis of an operating device for fluorescent lamps that operates resonantly. The power supply system voltage present at the two input terminals is rectified by the first bridge rectifier D1 and converted by a boost converter circuit, which often operates as a power supply system power factor correction circuit, to an intermediate circuit voltage of approximately 400 V at the intermediate circuit capacitor C5. By means of a half-bridge arrangement connected to the output terminals of the boost converter circuit, a rectangular voltage is generated from the boosted DC voltage. The half-bridge arrangement consists of two series-connected transistors Q1 and Q2. The inductor L1 and the blocking capacitor C1 form a resonant circuit. The DC voltage components of the signal are filtered by the blocking capacitor C1. The output current of the resonant circuit is rectified by the second bridge rectifier D2 and smoothed by a Pi filter including C3, L2 and C4. The semiconductor light sources are arranged in parallel with the capacitor C4. The parasitic capacitance CModule is situated between the LED module 5 and the grounding PE of the power supply system. Said parasitic capacitance is depicted for simplification between the cathode of the bottommost semiconductor light source or the negative terminal 56 of the LED module and the grounding PE. In practice it has been found that between the semiconductor light sources and the grounded heat sink high grounding currents of above 100 mA can flow via the parasitic capacitance CModule.
FIG. 3 shows this grounding current IPE in an oscillogram. The current ILED is the operating current through the LEDs. These high grounding currents give rise to severe electromagnetic interference. These currents are above the limit values of many correspondingly valid standards for luminaire arrangements in which the operating device for the LEDs and the LED module are spatially separated. As a result of the impermissibly high ground currents and the severe electromagnetic interference, this circuit topology cannot be used for the above-mentioned application.
A solution with a half-wave rectification that is known in the related art is shown in FIG. 4. A simple diode D3 is used instead of the second bridge rectifier. As a result, the cathode of the bottommost LED, and thus the negative pole 56 of the LED module 5, is always at ground. As a result of this measure, no significant interference current can form via the parasitic capacitance Cmodule.
As a result of the half-wave rectification, however, the diode D3 conducts only for one polarity of the resonant circuit including L1 and C2; therefore, a high reactive current in comparison with the LED current is necessary, which leads to higher losses and a low efficiency of the circuit arrangement of only 80% to 85%.
FIG. 5 shows an alternative with a voltage doubler circuit. This circuit includes two diodes D3 and D4, as a result of which a full-wave characteristic is provided. A current flows through D3 given a positive polarity of the resonance voltage at C2, and through D4 given a negative polarity thereof. The capacitors C3 and C5 provide for a current flow to ground. In comparison with the bridge rectifier in accordance with FIG. 2, however, the high-frequency voltage swing with respect to ground is very small. The advantages and disadvantages of the circuit are similar to the circuit having half-wave rectification in accordance with FIG. 4.